The measuring membrane is connected with the platform along an annular joint. Known materials for the joint are glass and active hard solders, or brazes. A pressure sensor joined with a glass solder is disclosed, for example, in German patent, DE 39420102B. An overview of the different approaches for manufacturing a joint with zirconium-nickel-titanium, active hard solder, or braze, is given in the as yet unpublished German patent applications DE 1020106365.9, DE 102009046844.7, DE 102009 054909.9 and the documents cited therein for the state of the art. The named materials basically fulfill their purpose, i.e. they serve to join the parts of pressure measurement cells with one another. However, use of these materials leads to limits for measuring cell design.
A basic cause therefor is the limited media resistance of the joint, from which, for pressure measurement cells of industrial process measurements technology, it follows that the joint needs to be protected from the measured medium. Insofar as the joint in the case of the, usually, cylindrical pressure measurement cells extends to their lateral surface, the protection of the joint results from the established, installed situation, wherein the pressure measuring cell is clamped axially in a sensor housing, wherein the pressure measuring cell end presses a sealing ring against an annular abutment surface, which surrounds a housing opening, through which the measuring membrane body is contactable at the end of the pressure measuring cell with a medium, whose pressure is to be measured.
Although the joint is, in this way, reliably protected from the medium, this requires an annular edge region of the end of the pressure measuring cell, thus the measuring membrane body, as bearing surface for the sealing ring, so that this edge region must be supported by the joint. With the trend being to make the diameter ever smaller, the relative proportion of this edge region on the end face grows, so that the deflectable part of the measuring membrane body available for the pressure measurement decreases disproportionately with the diameter of the pressure measuring cell. This sets limits for the miniaturization of pressure measurement cells.
Insofar as, with the decrease of the diameter of the pressure measuring cell, the relative area fraction of the joint compared with the deflectable area of the measuring membrane body disproportionately increases, differing thermomechanical properties between the material of the joint and the ceramic material of the measuring membrane body and of the platform gain with the miniaturization of the pressure measuring cell relatively in importance and can lead to warping and ultimately to measurement errors. These concerns discourage a miniaturization of the measuring cells.
Finally, reductions of the diameter of the pressure measuring cell lead in the case of pressure measurement cells with capacitive transducers to sinking capacitances. This can, indeed, be compensated theoretically by lessening the distance between platform and measuring membrane, respectively the electrodes mounted on the platform and measuring membrane. However, there are limits to this, since the material thickness of the established active hard solder, or braze, rings, with which the joint between platform and measuring membrane body is manufactured, is subject to a certain tolerance and is not manufacturable as thinly as desired.